This invention relates to a gas supply arrangement of a marine vessel and to a method of providing gas in a gas supply arrangement of a marine vessel.
The usage of gas as energy source in marine vessels is advantageous due to its efficient burning and low emissions. Usually gas is stored in liquefied form because less space is required for the storage that way.
The propulsion system of LNG (Liquified Natural Gas) tankers is usually powered by making use of the cargo. Storing of the gas in the tanker is arranged by using heat insulated cargo tanks into which an ullage space section and a liquid phase section are formed. The pressure in the cargo tanks is approximately at atmospheric pressure level and the temperature of the liquefied gas is about minus 163° C. Although the insulation of the cargo tank is extremely good, gradually increasing of the LNG temperature causes formation of so called natural boil-off gas. The boil-off gas must be removed in order to avoid extensive increasing of pressure in the cargo tanks, that is because the cargo tanks are very sensitive to pressure changes. The boil-off gas may be utilised in tanker's consumption devices like propulsion system. However, the amount of natural boil-off gas is not sufficient for providing all propulsion energy required in all circumstances and therefore the vessel must be provided with additional means for acquiring extra gas, so called forced boil-off gas.
For example in a patent publication FR 2722760 there is shown an arrangement in which liquid gas is supplied to a forced boiling vaporiser in which the liquid gas vaporises into gas form, which in turn may be combined with the natural boil-off gas stream.
EP 1348620 A1 shows a gas supply apparatus in which the natural boil-off gas is led to a compressor, which increase the pressure of the gas prior to feeding it to consumption via a feed line. Additionally, the apparatus also includes a forced boiling vaporiser in which the liquid gas previously pumped to the higher pressure is vaporised. In this arrangement the forced boiling gas portion is combined to the natural boil-off gas after the pressure of the natural boil-off gas has been increased. It is, however, somewhat complicated system requiring two parallel gas feed systems from the cargo tank to gas main supply line. That kind of an arrangement requires also somewhat complicated control system.
An objective of the invention is to provide a gas supply arrangement for a marine vessel, which solves the above mentioned and other problems of the prior art. It is also an objective of the invention to provide a straightforward and reliable arrangement and method for a marine vessel with liquefied gas tank, which provides even pressure at the supply line and reliable gas supply for consumption devices of the vessel.